1. Field of the Invention
The present invention relates to an image forming apparatus such as copying machine, printer, or facsimile using an electrophotographic process.
2. Related Background Art
Hitherto, various image forming apparatuses using the electrophotographic process have been proposed or embodied. For example, there are a system for transferring toner images formed on a photosensitive drum so as to sequentially superpose to a transfer material that is borne and conveyed by a transfer drum or a transfer belt, and a system for primarily transferring toner images formed on the photosensitive drum so as to sequentially superpose to an intermediate transfer belt or an intermediate transfer drum and for secondarily transferring the toner images on the intermediate transfer belt or the intermediate transfer drum to the transfer material. The image forming apparatuses using the two systems will now be described hereinbelow.
The image forming process of the image forming apparatus using the former system (transfer drum) will be simply explained. First, a photosensitive drum is rotated and a charger uniformly charges the surface. Subsequently, a laser beam modulated by an image signal of a first color, for example, magenta of original is irradiated onto the photosensitive drum to form an electrostatic magenta latent image on the photosensitive drum. A magenta developing device develops the electrostatic latent image to form a magenta toner image of the first color on the photosensitive drum.
On the other hand, a transfer material received in a sheet feed cassette is conveyed to a transfer drum by a registration roller or the like. Simultaneously with the conveyance of the transfer material, an adsorption roller is thrust to the surface of the transfer drum and, an adsorption charger adsorbs and charges the transfer material from the rear surface side of the transfer material to electrostatically adsorb the transfer material to the transfer drum. The transfer drum is rotated synchronously with the photosensitive drum. The magenta toner image formed on the photosensitive drum is transferred to the transfer material borne on the transfer drum by a transfer charger. The transfer drum continuously rotates as it is to prepare the next transfer of a cyan toner image of the second color.
The cyan toner image of the second color is formed on the photosensitive drum. The cyan toner image is transferred to the transfer material borne on the transfer drum so as to be superposed onto the magenta toner image. The similar image forming process is repeated with respect to yellow and black of the third and fourth colors, thereby obtaining a fullcolor image formed by superposing the fullcolor toner images of magenta, cyan, yellow, and black on the transfer material.
The transfer material on which the four color toner images have been transferred is separated from the transfer drum and conveyed to a fixing device. The fixing device heats and pressurizes the toner images and transfer material by a fixing roller and a pressure roller to mix the respective colors of the toner images and fix the images to the transfer material, thereby forming a fullcolor print image. The transfer material is discharged out of the apparatus.
An image forming process of the image forming apparatus using the latter system (intermediate transfer drum) will be simply described. A photosensitive drum is rotated at a predetermined peripheral speed. A charger uniformly charges the surface of the photosensitive drum. A laser beam is scanned to expose by an exposing device, an electrostatic latent image of the first color is formed on the photosensitive drum, and the latent image is developed by a developing apparatus. The developing apparatus has four developing devices for a yellow toner, a magenta toner, a cyan toner, and a black toner therein. The developing device for yellow develops the first color electrostatic latent image on the photosensitive drum to visualize as a yellow toner image.
The formed yellow toner image is electrostatically transferred to the intermediate transfer drum in a primary transferring portion where the intermediate transfer drum is come into contact with the photosensitive drum (primary transfer). For the photosensitive drum on which the primary transfer was completed, a cleaner removes the toner remained on the surface and, after that, the drum is used for the next color image formation.
Similarly, the charge by the charger and the exposure of the laser beam are performed to the photosensitive drum to form an electrostatic latent image of the second color. The developing device for magenta develops the latent image on the photosensitive drum to form a magenta toner image on the photosensitive drum. The magenta toner image is transferred to the intermediate transfer drum so as to be superposed to the yellow toner image.
The above process is also repeated with respect to cyan and black to sequentially superpose and transfer the toner images to the intermediate transfer drum. Consequently, a color image obtained by superposing the four color toner images of yellow, magenta, cyan, and black on top of one another is formed on the intermediate transfer drum.
After that, a secondary transfer charger in a separating state is come into contact with the surface of the intermediate transfer drum. In a secondary transferring portion where the intermediate transfer drum is come into contact with the secondary transfer charger, the four color toner images on the intermediate transfer drum are collectively transferred to the surface of the transfer material which is conveyed to the secondary transferring portion at a predetermined timing (secondary transfer).
The transfer material on which the four color toner images have been transferred is conveyed from the intermediate transfer drum to a fixing device, the toner images are fixed by a heat roller or the like to form a fullcolor permanent image, and after that, the transfer material is discharged out of the image forming apparatus.
However, in the image forming apparatus using the former system as mentioned above, since the toner image transfer to the transfer material is repeated, a phenomenon called counter-transfer that the toner image which has already been transferred to the transfer material is offset onto the photosensitive drum at the time of the transfer process for the next color toner image can occur.
For example, in case of forming a blue image, a solid magenta image is first transferred to the transfer material and a solid cyan image is subsequently multi-transferred to it. At the time of the subsequent transfer process, for instance, the transfer processes for yellow and black, no toner to be transferred exists on the corresponding position on the photosensitive drum. In such a state, when the toner image on the surface of the transfer material is close to or come into contact with the surface of the photosensitive drum, such a phenomenon that a part of the toner of the toner image on the transfer material (in this case, it denotes mainly a cyan toner image), namely, cyan toner image is counter-transferred onto the photosensitive drum occurs. Consequently, the density of the cyan toner image deteriorates in the portion where the cyan toner has been counter-transferred, the magenta color of the magenta toner image coated under the cyan toner image appears, and color heterogeneity is generated, so that remarkable deterioration of the image quality is caused.
The phenomenon remarkably occurs when the charge amount of the toner is changed in association with the change in temperature and humidity or the charge amount retainable per unit area of the transfer material is changed in accordance with the kind of transfer material or change in temperature and humidity. Particularly in recent years, as the particle diameter of the toner becomes smaller in order to realize a high picture quality, the charge amount per toner particle 5 is reduced but the total charge amount of the toner image can increase. In association with such a tendency, a transfer current or a transfer voltage necessary to transfer the solid image becomes larger. The increase in transfer current or transfer voltage also exerts an adverse influence to the above-mentioned counter-transfer phenomenon.
In the image forming apparatus using the latter system as mentioned above, in the secondary transfer process, a large amount of toner stacked by an amount corresponding to four colors on the intermediate transfer drum is collectively transferred to the transfer material. At that time, when the absolute value of the charge amount of toner to be transferred is relatively small, the secondary transfer is favorably performed.
However, depending on the ambient atmosphere where the image forming apparatus is used or the kind of transfer material, although it is remarkably recognized particularly in a low humidity environment, there is such a problem that when an enough amount of toner is transferred from the intermediate transfer drum to the transfer material and retained in a state where the absolute value of the toner charge amount is extremely increased, an image failure like a mark due to abnormal discharge occurs. It is thought that the phenomenon is caused because the retainable charge amount per unit area of the transfer material differs depending on the kind of transfer material.
In the case where the charge amount of toner of at least one color among the four color toners used for the image formation is smaller than those of the other colors, in order to uniform the transfer performances of the four color toners, the re-charge is performed to the toner image on the intermediate transfer drum by a corona charger or the like after the primary transfer is completed and before the secondary transfer is started, and the charge amounts of four colors are equalized to uniform the transfer performances in many cases. In this manner, the absolute value of the toner charge amount is larger than that in the preceding state and, in a manner similar to the above, an image failure like a mark due to abnormal discharge occurs at the secondary transferring time.